The invention relates to a device in optical representation systems of a motion picture camera.
For visually judging an image or subject which is to be taken, more particularly for judging the sharpness of the image, so-called focusing screens are known in the form of matt discs or fiber plates in the viewfinder or video beam path of a motion picture film camera. Matt discs are matted for this purpose either through a grinding process or through a chemical etching process. Fibre plates on the other hand are produced from a bundle of individual parallel-aligned light-conductive fibres.
Both types of focusing screens have a structure which is obtrusive, particularly in the case of small-format images as are customary in film pictures. These structures can indeed be reduced in the case of a matt disc by using a particularly fine grinding particle size during the grinding process, but the matt disc can become too transparent so that it is difficult to judge the optimum focus setting and the brightness of the image becomes uneven. The same drawbacks occur in the case of matt discs which are coated on their matt surface with a transparent lacquer in order to reduce the graininess.
Fibre plates have the advantage over matt discs in that their structure is smaller, however they are more expensive than matt discs and particularly when used with smaller apertures of the imaging lens they can detect a regular mostly honeycombed structure which destroys the artistic judgement of the image or subject.
The structures which are to be seen in the focusing discs can thus be recognised with a clarity which is greater the more the object lens of the film camera is faded out. Matt discs thereby act to provide a blackened soot effect.
In order to reduce the troublesome structures which appear, i.e. the graininess and honeycombed structure a translucent image screen is known From German Patent Specification 2 016 183 which consists of two plates with an elastic transparent filler compound in between which forms a layer which adjoins at least in parts both scattering surfaces. The plates are connected to a drive device which moves the plates relative to each other at a frequency which is clearly above the temporal resolution capacity of the eyes of about 60 Hz. By means of such an arrangement it is possible to eliminate or at least significantly reduce the appearance of graininess and the honeycombed structure.
For an extensive visual judgement of the image quality however it is necessary to provide on a translucent focusing disc an image field marking which indicates the outer limits of the image or subject which is to be recorded and which is provided in the form of format markings on the surface of the focusing screen, In order that the image field markings can be better detected if the amount of light passing through the camera lens is low the format marking can be illuminated directly or indirectly through a light source mounted in the camera so that even with unfavourable light conditions the markings remain visible to the operator in the viewfinder of the camera.
However reducing or eliminating the structures in the case of matt discs and fiber plates by moving the focusing screen leads to smudging of the image field marking as a result of the moved focusing screen and thus leads to faulty detection of the image field limits. Also movement of the combination of focusing screens formed from plates cemented together leads to faulty judgement of the image quality, i.e. the image field definition and image sharpness, so that either an increased detectability of the graininess or a lacking detectability of the image field marking has to be accepted.
From U.S. Pat. No. 5,802,406 a photographic system is known for a camera viewfinder with a focusing screen which is mounted in the representation plane of the object lens and has a semi-transparent section which encloses a rectangular completely light-transparent photographic area surrounded by a frame on which the recording subject is represented. A circular photometric area is arranged in the completely transparent photographic region and reproduces the area analyzed by a photometric device The transparent photographic area, the photometric area and a marking indicating the center of the transparent photographic area are arranged as image field markings on the focusing screen.
From CH A 617 274 a camera viewfinder is known which is mounted in the reflected beam path of a rotating aperture which alternately releases the recording beam path for the exposure of the moving film and deflects it in the viewfinder beam path. The camera viewfinder contains a matt disc which has on one side a matt surface and on the other side a line pattern of light-reflecting material for the image field definition. The viewfinder contains an eyepiece and a beam divider which reflects the recording beam path passing through the matt disc to the eyepiece. Furthermore a lighting arrangement is provided in the viewfinder by means of which light is directed through the beam divider to the light-reflecting material whereby the light reflected by the light-reflecting material is sent back to the beam divider and is supplied from here to the eyepiece.
The object of the present invention is therefore to provide a device in optical representation systems of a film camera of the type mentioned at the beginning which improves the image quality on focusing screens and which enables optimum judgement of an image represented on the focusing screen (matt disc or fibre plate) without soot effects, troublesome graininess or honeycombed structure, guarantees satisfactory recognition of the image field definitions and avoids interference and thus ensures a satisfactory image viewing without troublesome side-effects.
This is achieved according to the Invention through a device having the features of claim 1.
The solution according to the invention improves the image quality on focusing screens in optical representation systems of a film camera, guarantees an optimum judgement of an image represented on the focusing screen (matt disc or fibre plate) without soot effects, troublesome graininess or honeycombed structure, as well as a satisfactory detection of image field definitions, avoids interference and thus guarantees a satisfactory recognition of image field definitions without troublesome side effects.
The solution according to the invention is particularly suitable for video reflecting devices in motion picture film recording cameras since the Improvement which can be achieved with the invention in the image quality of a focusing screen can indeed be seen in a normal viewfinder but is most clearly apparent however in a video reflecting device and avoids interference and the appearance of flickering effects.
In order to avoid parallax errors according to a further feature of the invention the distance between the movable disc and stationary disc is minimal.
An advantageous development of the solution according to the invention is characterised in that the movable disc is mounted so that it is movable solely in the plane of the disc.
By precisely guiding the movable disc in its plane a situation is avoided where the camera lens is no longer correctly focused or falsely set as a result of a deviation of the disc in the beam direction.
In order to eliminate completely the visual impression of graininess on a matt disc or honeycombed structure on a fibre plate according to a further feature of the solution according to the invention the drive device moves the movable disc with a frequency which is dependent on the image frequency of the film or video camera. Since the motion frequency of the moved disc is set In relation to the Image frequency Interference phenomena can be avoided and thus a satisfactory image consideration can be ensured without troublesome side effects.
As an altemative or in addition to this the drive device which is connected to the movable disc can move the movable disc with a frequency which is dependent on the brightness of the image or subject being recorded. Since the brightness influences the opening of the camera aperture and with a little opened aperture the so-called soot effect appears more strongly, with a wide opened recording aperture the motion frequency of the moved disc can be reduced without having to accept losses in the quality of the image consideration.
Preferably the movable disc is mounted movable in the plane of the disc and executes an eccentric more particularly irregular movement. The irregularity in the movement of the movable disc avoids on the one hand reversal points in an oscillating movement where the graininess is again to be observed, and on the other hand renewed detection of grains cannot occur as a result of periodic recurrence of image spots at the same place on the matt disc or fibre plate.
In an advantageous development of the solution according to the invention the movable disc executes a movement which results from superimposing two periodic movements running in directions at right angles to each other, i.e. describes so-called Lissajous figures.
As an alternative to this the movable disc executes a movement which is similar to the movement of an arm of a four-armed gearbox.
Furthermore the motion frequency of the drive device can be coupled timewise or speedwise with the image frequency of the film camera or the video frequency of a video reflecting device of the film camera whereby preferably the motion frequency is In a not whole number ratio to the image frequency of the film camera or video frequency of the video reflecting device.
This measure avoids the risk of interference occurring or periodically recurring settings and thus optimises the image judgement without disturbing side effects.
As regards the type of movement of the movable disc it is immaterial whether the movable disc carries out only part of a complete movement or even several complete movements within one time period in which light is available in the viewfinder beam path, so long as the focusing screen is moved over a distance from one grain to the next on a matt disc or from one fibre to the next on a fibre plate within the perception limits of the human eye or a video reflecting device mounted where required in the viewfinder path.
An advantageous development of the solution according to the invention is characterised in that the movable disc is moved at least at times by the drive device.
Since the image impression is particularly relevant in the standby mode of the film camera, in one development of the solution according to the invention the movable disc is moved in the standby mode of the film camera and is stopped during the run mode of the film camera so that camera noises are minimized. This is particularly advantageous in the case of so-called sound-sync cameras.
A further development of the solution according to the invention is characterised in that the movable disc is moved independently of the operating stage of the film camera with a high motion frequency, which is a particularly satisfactory solution in the case of so-called non-sound-sync cameras.
The drive device for the movable disc can be formed in different ways. Thus there is the possibility of connecting the drive device to the film transport mechanism of the film cameras through a gearing unit or to couple the drive device to an aperture drive for a rotating reflex aperture of a film camera.
Furthermore the drive device can consist of at least one eccentric motor connected directly or indirectly to the movable disc.
As an alternative to the eccentric motor it is possible to provide for example selectively a piezo element connected to a side edge of the movable disc, a push rod or a lift magnet as the drive device or to mount a movable disc connected to metal or magnetic parts in an inductive or capacitive field so that through corresponding excitation the movable disc executes movements which are predetermined through the field changes.
Where the focusing screen is comprised of two parallel translucent plates which are connected together by a cement material the drive device engages on this sandwich-structure focusing screen whilst a focusing screen supporting the image field markings is mounted stationary in the image field area of the movable disc.
In a further embodiment the movable disc and the stationary disc with image field marking can be formed from parallel translucent plates between which an optical medium is located in the form of a gas, fluid or elastic solid bodies.
In this embodiment the drive device engages on the movable disc whilst the other disc is mounted locally fixed. The discs can execute relative movements to each other through the optical medium.
This solution has proved particularly space saving and by integrating the measures which reduce the graininess the problem posed can be solved with an extremely compact effective device for improving the image quality on focusing screens in optical representation systems of a film camera or a video reflecting device of a film camera.
The connection of the plates or discs is carried out substantially over the entire surface areas of the plates with a translucent optical medium. The refraction at the matted surface of the plates hereby takes place not opposite air but opposite the optical medium and the scatter behavior of the matted surfaces can be influenced through the corresponding choice of optical medium and can be tuned in optimum manner to the relevant use.
Preferably the material of the plates or discs has an optical medium refraction index which is higher by at least a factor of 1.1, and more particularly by a factor 1.15 to 1.3.
In order to be able to detect the image field marking of the stationary disc even in poor lighting conditions a light source is provided inside the camera to beam on the stationary disc.
The sequence of the optically active elements of the device according to the invention is basically in any order for the desired effect like the relevant sides of the optically active elements, but there are specific advantages depending on the sequence.
Thus in a first arrangement the movable disc faces the camera lens with its non-matted side, whilst the stationary disc faces the matted side of the movable disc with its format marking. With this arrangement the shortest possible distance is achieved between the plane of the matt disc and the format marking.
In a second arrangement the movable disc faces the camera lens with its matted side and the stationary disc faces the non-matted side of the movable disc with its format marking.
With this arrangement the distance between the plane of the matt disc and the format marking is indeed greater by the thickness of a glass plate but the bearing dimension for this, i.e. the distance between the lens fixing and the image window of the camera, can be measured more easily.
Finally the stationary disc which faces the camera lens with its normal glass side can be turned so that its side supporting the format marking faces the matted side of the movable disc. This makes it possible to change the format markings more easily.